Twenty-year-old Tyler Peterson had gone to the party early Sunday to make amends with his ex-girlfriend, a friend of Peterson's told The Milwaukee Journal Sentinel.

Peterson lost control after people called him a "worthless pig," Mike Kegley told the paper.

Peterson left and got a police-style AR-15 rifle from his truck, forced his way back into the apartment and fired about 30 rounds at about 2:45 a.m. (3:45 a.m. ET). Six people were killed; one person survived and is hospitalized, Attorney General J.B. Van Hollen said at a Monday news conference.

Peterson was killed in a shootout with law officers Sunday afternoon after negotiations for his surrender failed, officials said. The town's mayor said Tyler was killed by a SWAT team sniper.

The dead and wounded were all students or graduates of Crandon High School, and Peterson was a graduate of the school, which has a little more than 300 students.

Witnesses said the victims ranged in age from 14 to 20, and one was apparently Peterson's former girlfriend.

Peterson's family, in a statement read by Bill Farr, a pastor, expressed condolences to the victim's relatives and said they could not find any reason for the killings.

"We are grieving for your losses. We are very sorry for what has happened. This huge tragedy has deeply affected everyone, including us. We also feel a tremendous amount of guilt and shame for the horrible acts Tyler committed. This is not the Tyler we knew and loved," the statement read.

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"I don't want to believe it. I'm waiting for somebody to wake me up," she said. "She's only 14 -- she'll be 15 next month; she's just starting to live. And the sad thing is who killed her -- a cop. Cops are supposed to always protect you, I thought, and it's one who took my daughter and how many other people's lives."

It was the high school's homecoming weekend.

Friends of the victims said Peterson also worked part time as a Crandon police officer.

Residents near the scene of the shooting told the Associated Press it was hard to accept that a police officer was the shooter.

"The first statement we said to each other was, 'How did he get through the system?' " David Franz told the AP. "How do they know somebody's background, especially that young? It is disturbing, to say the least."

The town's schools were closed Monday, with grief counselors available to students, said Superintendent Richard Peters.

"This was the kind of scenario where every small town in the USA says, 'This could never happen here,' " Peters said.

Crandon, a town of about 2,000 people, is 220 miles north of Milwaukee, Wisconsin.

Crandon Mayor Gary Bradley said the town will work together to get over the tragedy.

"There's a lot of people weeping and gnashing their teeth and the emotions are very raw right now," Bradley said. "But we'll rebuild brick by brick."

Forest County Sheriff Keith Van Cleve called the situation "very difficult" for his deputies and the community.

Karly Johnson, 16, told the AP she knew the shooter.

"He was nice. He was an average guy -- normal. You wouldn't think he could do that," Johnson said, adding that Peterson had helped her in a class and had graduated with her brother, according to the AP.

The state attorney general's office will investigate the case, Van Cleve said.

Kevin St. John, a spokesman for the state Department of Justice, said the agency's criminal investigation unit routinely investigates cases of a "statewide or significant nature." E-mail to a friend

CNN's Susan Roesgen and Katherine Wojtecki contributed to this report.

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